Momo's mention of HäT's Tips page makes me think that you could also look at the websites of other current makers of soft plastic figures, to see if they make recommendations for painting them. 

Here are some:

Armies in Plastic: https://www.armiesinplastic.com/

Engineer Basevich (Facebook page, not website): https://www.facebook.com/Toy-soldiers-Engineer-Basevich-322160358341602/

Conte: http://www.conteco.com/

Another place to check is at Hobby Bunker, a shop in Malden, Massachusetts. Owner Matt Murphy is a gamer himself, and carries a large selection of plastic figures.  You might find tips there, or even contact Matt and see if he has advice.